# std::memcpy

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< cpp‎ | string‎ | byte

Defined in header `<cstring>` |  |
void* memcpy( void* dest, const void* src, std::size_t count ); |  |

Copies count bytes from the object pointed to by src to the object pointed to by dest. Both objects are reinterpreted as arrays of unsigned char.

If the objects overlap, the behavior is undefined.

If either dest or src is an invalid or null pointer, the behavior is undefined, even if count is zero.

If the objects are potentially-overlapping or not TriviallyCopyable, the behavior of `memcpy` is not specified and may be undefined.

### Parameters

dest | - | pointer to the memory location to copy to
src | - | pointer to the memory location to copy from
count | - | number of bytes to copy

### Return value

dest

### Notes

`std::memcpy` may be used to implicitly create objects in the destination buffer.

`std::memcpy` is meant to be the fastest library routine for memory-to-memory copy. It is usually more efficient than std::strcpy, which must scan the data it copies or std::memmove, which must take precautions to handle overlapping inputs.

Several C++ compilers transform suitable memory-copying loops to `std::memcpy` calls.

Where strict aliasing prohibits examining the same memory as values of two different types, `std::memcpy` may be used to convert the values.

### Example

Run this code

    #include <cstdint>
    #include <cstring>
    #include <iostream>

    int main()
    {
        // simple usage
        char source[] = "once upon a daydream...", dest[4];
        std::memcpy(dest, source, sizeof dest);
        std::cout << "dest[4] = {";
        for (int n{}; char c : dest)
            std::cout << (n++ ? ", " : "") << '\'' << c << "'";
        std::cout << "};\n";

        // reinterpreting
        double d = 0.1;
    //  std::int64_t n = *reinterpret_cast<std::int64_t*>(&d); // aliasing violation
        std::int64_t n;
        std::memcpy(&n, &d, sizeof d); // OK

        std::cout << std::hexfloat << d << " is " << std::hex << n
                  << " as a std::int64_t\n" << std::dec;

        // object creation in destination buffer
        struct S
        {
            int x{42};
            void print() const { std::cout << '{' << x << "}\n"; }
        } s;
        alignas(S) char buf[sizeof(S)];
        S* ps = new (buf) S; // placement new
        std::memcpy(ps, &s, sizeof s);
        ps->print();
    }

Output:

    dest[4] = {'o', 'n', 'c', 'e'};
    0x1.999999999999ap-4 is 3fb999999999999a as a std::int64_t
    {42}

### See also

memmove | moves one buffer to another (function)
memset | fills a buffer with a character (function)
wmemcpy | copies a certain amount of wide characters between two non-overlapping arrays (function)
copy | copies characters (public member function of `std::basic_string<CharT,Traits,Allocator>`)
copycopy_if (C++11) | copies a range of elements to a new location (function template)
copy_backward | copies a range of elements in backwards order (function template)
is_trivially_copyable (C++11) | checks if a type is trivially copyable (class template)
C documentation for memcpy

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